The S wave is the first downward deflection of the QRS complex that occurs after the R wave. However, a S wave may not be present in all ECG leads in a given patient.

In the normal ECG, there is a large S wave in V1 that progressively becomes smaller, to the point that almost no S wave is present in V6. A large slurred S wave is seen in leads I and V6 in the setting of a right bundle branch block.

The presence or absence of the S wave does not bear major clinical significance. Rarely is the morphology of the S wave discussed.

In the setting of a pulmonary embolism, a large S wave may be present in lead I — part of the S1Q3T3 pattern seen in this disease state. At times, the morphology of the S wave is examined to determine if ventricular tachycardia or supraventricular tachycardia with aberrancy is present; this is discussed elsewhere.